The present invention relates to an earth cutting device for excavation purposes and is particularly, although not exclusively, concerned with excavating hard rock. It will be convenient therefore, to describe the invention in relation to that application, although it is to be appreciated that the invention could have wider application.
Traditionally, excavation of hard rock in the mining and construction industries, has taken one of either two forms, namely explosive excavation, or rolling edge disc cutter excavation. Explosive mining entails drilling a pattern of holes of relatively small diameter into the rock being excavated, and loading those holes with explosives. The explosives are then detonated in a sequence designed to fragment the required volume of rock for subsequent removal by suitable loading and transport equipment. The explosives are detonated once all personnel are evacuated from the excavation site and the explosive process is repeated cyclically, until the required excavation is complete.
The use of explosives for excavation is known to be dangerous, while it is also environmentally unfriendly and results in damage to the country rock, with the result that clearing of loosened rock pieces and the erection of supports for the excavated surfaces is both dangerous and difficult. Additionally, the cyclical nature of the process and the violent nature of the rock fragmentation has to date, prevented automation of the explosive process, so that the modem requirement for continuous operation and increased production efficiency has not been met. Moreover, the relatively unpredictable size distribution of the rock product formed, complicates downstream processing.
Mechanical fragmentation of rock eliminating the use of explosives, has already been achieved and is well known through the use of rolling edge-type disc cutter technology. This technology has facilitated automation of the excavation process including the benefit of remotely controlled excavation machinery. However, rolling edge cutters require the application of very large forces in order to crush and fragment the rock under excavation. For example, the average force required per cutter is in the order of 50 tonnes and typically, peak forces experienced by each cutter are more than twice than this. It is common for multiple cutters to be arranged to traverse the rock in closely spaced parallel paths, and 50 cutters per cutting array is common. Cutting machinery of this kind can weigh upwards of 800 tonnes, thereby requiring electrical power in the order of thousands of kilowatts for operation. As such, that machinery can only be economically employed on large projects, such as water and power supply tunnels. Additionally, the excavation carried out by such machinery is limited to a cross-section which is circular.
It is an object of the invention to overcome, or at least alleviate one or more of the disadvantages associated with prior art cutting devices. It is a further object of the invention to provide a cutting device of a rotary cutting type, that provides improved rock removal from a rock face and which is relatively economical to manufacture and operate.
A rock excavating or cutting device according to the present invention includes a disc cutter, and is characterised in that the disc cutter is driven to move in an oscillating and nutating manner. The disc cutter is driven to move in this manner about separate oscillating and nutating axes, which are angularly offset from one another and intersect at a point ahead of the disc cutter. The magnitude of nutating movement is directly proportional to the angle of offset between the respective axes and generally that angle will be relatively small, such that the point of intersection between the axes is a relatively long way ahead of the disc cutter. In some arrangements, the point of intersection will approach infinity such that the amount of nutating movement is very small. Preferably, the disc cutter is caused to oscillate and nutate sinusoidally through a relatively small amplitude and at a very high frequency, such as about 3000 RPM.
The motion by which the disc cutter is driven, is such as to cause tensile failure of the rock, so that chips of rock are displaced from the rock surface under attack by the disc cutter. Here, the invention differs from rolling edge disc cutters, which apply force normal to the rock face to form lateral cracks that produce rock chips.
The force required to produce a tensile failure in the rock to displace a rock chip according to the device of the invention, is an order of magnitude less than that required by the known rolling edge disc cutters to remove the same amount of rock, so that the device of the invention is far more efficient in respect of energy requirements. Additionally, the device of the invention produces relatively little dust.
The device of the invention employs a reaction mass of sufficient magnitude to absorb the forces applied to the rock by the disc cutter during each cycle of oscillation and nutation, with minimum or minor displacement of the device, or the structure supporting the device. Because the device applies a load suitable to cause tensile failure of the rock, instead of crushing the rock, the force applied to the rock is substantially reduced, such that a corresponding reduction in the required reaction mass compared to known rock excavation machinery can also be adopted. The device of the invention as mounted to the support structure is preferably arranged that the reaction mass can absorb the cyclic and peak forces experienced by the disc cutter, while the support structure provides a restoring force relative to the average force experienced by the disc cutter.
The disc cutter of the cutting device preferably has a circular, rock engaging periphery, which is formed of a wear resistant material, such as hardened steel or tungsten carbide. Alternatively, the disc cutter can include a plurality of cutting tips, preferably of tungsten carbide, which are fixed to the circular rock engaging periphery thereof. Alternatively, the disc cutter can include a removable cutting disc that likewise is formed to have a circular rock engaging periphery of a wear resistant material, such as that described above.
The periphery of the disc cutter is arranged to be rotatable relative to the oscillating and nutating movement thereof, so that the periphery can roll against the rock surface under attack. In this manner, all parts of the cutting periphery edge are progressively moved out of contact with the rock and allowed to cool, and wear is evenly distributed. Because the contact force is relatively low, the wear rate is reduced compared to the rolling edge type of cutter.
The oscillating movement of the disc cutter can be generated in any suitable manner. In a preferred arrangement, the disc cutter is mounted for rotary movement on a drive shaft that includes a driven section which can be driven by suitable driving means and a mounting section on which the disc cutter is mounted. The axis about which the driven section rotates is angularly offset from the axis of the mounting section and in this arrangement, the disc cutter can move, as required, in a nutating manner simultaneously as it oscillates.
In a preferred arrangement, the disc cutter is mounted on one end of the shaft, which end comprises the mounting section and which extends from the shaft at an angle offset from the longitudinal axis of the shaft. The offset end may be formed integral with the shaft, or may be attached thereto and the end may include means to attach the disc cutter thereto. Those means allow for relative rotary movement, between the disc cutter and the mounting. The disc cutter may for example, be mounted on the mounting section by bearings, such as tapered roller bearings, to allow relative rotation therebetween.
The device of the invention can operate to cut or excavate very hard rock, with greatly reduced applied force and much higher output per disc cutter, while using less power per unit volume of rock removed. Thus the device can be mounted on a vehicle of significantly reduced weight and cost, compared, for example, to rolling edge disc cutters, while providing much greater flexibility in the geometry of excavation.
The cutting device of the invention is not restricted to a single disc cutter, but can include more than one. For example, the cutting device may include three disc cutters arranged along the same plane, but angled at approximately 45xc2x0 to each other. Such an arrangement can produce a cut face of a particular shape, while the speed at which rock is removed is greatly increased. In this arrangement, each of the three disc cutters can be driven by the one drive means, or they may be driven by separate drive means. The use of multiple disc cutters is particularly useful for long wall operations.
The device of the invention typically requires substantially reduced applied forces relative to known rock excavating machinery. A reduction at least in respect of normal forces, in the order of one tenth is envisaged. Such low forces facilitates the use of a support structure in the form of an arm or boom, which can force the edge of the disc cutter into contact with the rock at any required angle and to manipulate the position of the disc cutter In any direction. In particular, in relation to long wall mining, the disc cutter, or array of disc cutters, may be mounted to traverse the length of the long wall face and to be advanced at each pass. Advantageously, the invention provides for entry of the disc cutter into the rock face from either a previously excavated drive in a long wall excavation, or from pre-bored access holes, or by attacking the rock at a shallow angle to the face until the required depth for the pass is achieved. With the disc cutter mounted on a movable boom, the disc cutter can be moved about the rock face to excavate that face at any desired geometry.
In still a further arrangement, a pair of disc cutters may be mounted on separate booms and the disc cutters are swept in an arc across the rock face, continually removing successive layers of rock from the face, and forming a cusp between adjacent concave sections. The cusp provides an entry point for the disc cutter on the return pass thereof.
The cutting device of the invention is suitable for a range of cutting and mining operations and machinery, such long wall mining, mobile mining machines, tunnelling machines, raise borers, shaft sinkers and hard rock excavation generally.